1. Field of the Invention
The present invention relates to a cellulose acylate film, which is excellent in the wet heat durability of the retardation in the in-plane direction and in the thickness direction thereof and of which the haze level is sufficiently low, and to a method for producing it. Precisely, the invention relates to a cellulose acylate film which has been improved in point of the wet heat durability of the optical properties thereof by adding an additive having a specific structure thereto, and which is useful in liquid crystal display devices and which hardly whitens (the haze is low), to a method for producing it, and to a polarizer and a liquid crystal device produced by the use of the cellulose acylate film.
2. Description of the Related Art
In general, a liquid crystal display device comprises a liquid crystal cell, an optically-compensatory film, and a polarizing element. The optically-compensatory film serves to cancel image coloration and to enlarge a viewing angle, including a stretched birefringent film and a film produced by coating a transparent film with a liquid crystal. For example, Japanese Patent No. 2587398 discloses a technique of applying an optically-compensatory film prepared by applying a discotic liquid crystal to a triacetyl cellulose film and aligning and fixing it thereon, to a TN-mode liquid crystal cell to thereby enlarge a viewing angle.
However, for a liquid crystal display device for TVs that are expected to be watched at various angles on a large-size panel, the requirement in point of the viewing angle dependence thereof is severe, and still could not be on a satisfactory level even though the above-mentioned technique is applied thereto. Accordingly, others than TN-mode liquid crystal display devices, such as IPS (in-plane switching) mode, OCB (optically compensatory bend) mode and VA (vertically aligned) mode devices are now under investigations. In particular, VA-mode devices have a high contrast and the production yield thereof is relatively high, and therefore they are now being in the mainstream of liquid crystal display devices for TV.
As the material of the polarizing element that is indispensable in a liquid crystal display device, in general, polyvinyl alcohol (hereinafter this may be referred to as “PVA”) is mainly used. A PVA film is monoaxially stretched and then colored with iodine or a dichroic dye, or after colored, it is stretched, and thereafter the resulting film is crosslinked with a boron compound to have a polarizing ability, and is used as a polarizing element.
For use that requires optical isotropy as in protective films for polarizers, cellulose acylate films are generally used. This is based on the characteristics thereof in that cellulose acylate films have a higher optical isotropy (having a lower retardation) as compared with other polymer films.
On the other hand, an optically-compensatory film (retardation film) in liquid crystal display devices is required to have an optical anisotropy (a high retardation) contrary to the above. Especially an optically-compensatory film for use in VA mode devices is required to have an Re (Re indicates the in-plane retardation of the film.) from 30 to 100 nm and an Rth (Rth indicates retardation in the thickness direction of the film.) from 80 to 300 nm. Accordingly, as the optically-compensatory film, heretofore generally used are synthetic polymer films having a high retardation, such as polycarbonate films and polysulfone films.
Specifically, the general principle of optical members for use in liquid crystal display devices is that synthetic polymer films are used in case where the polymer films are required to have an optical anisotropy (a high retardation), but cellulose acylate films are used in case where the films are required to have an optical isotropy (a low retardation).
In EP-A 911656, proposed is a cellulose acetate film having a high retardation which is applicable also to use that requires an optical anisotropy, contrary to the conventional general principle. In this proposal, an aromatic compound having at least two aromatic rings, especially a compound having 1,3,5-triazine rings is added to a cellulose triacetate film and the film is stretched to thereby realize a high retardation of the film. In general, cellulose triacetate is a hardly-stretchable polymer material, and it is known that the birefringence of the film is difficult to increase; however, the additive in the film is also oriented therein through the stretching treatment of the film, whereby the birefringence of the stretched film can be increased and a high retardation of the resulting film is thereby realized. The film can serve also as a protective film of a polarizer, and therefore has an advantage in that the necessary member films constituting a liquid crystal display device can be reduced, and inexpensive and thin-body liquid crystal display devices can be provided.
The methods disclosed in EP-A 911656 and JP 2587398 is useful in that thin liquid crystal display devices are manufactured economically.
Recently, use of liquid crystal display devices is increasing more and more, and the devices for outdoor use and those for in-car use are much increasing. Accordingly, liquid crystal display devices favorable for use in wet heat environments are being required.
For example, as a method for improving the moisture permeability of a cellulose acylate film in use in wet heat environments, a method of increasing the amount of the additive to the film is proposed (See JP-A 2002-22956 and JP-A 2001-354802)).
Further recently, it has become desired to improve the durability of the optical properties of the film in long-term use in wet heat environments, but conventional additives are ineffective for improving the film.
On the other hand, there is known a method of improving the properties of films by adding styrene/maleic anhydride additives to various resins. For example, there is disclosed an example of trying improving the wavelength dispersion characteristics of retardation of films by adding a styrene/maleic anhydride additive to a norbornene resin (JP-A 2001-337222); however, nothing is referred to therein relating to the improvement in the retardation of the films and to the usefulness of the styrenic additive under high-temperature high-humidity conditions. Also disclosed is an example of trying relieving the humidity dependence of films by adding a styrene/maleic anhydride additive to a cellulose acylate resin and further adding an antioxidant thereto (JP-A 2007-304376); however, nothing is referred to therein relating to the optical properties of the films. A styrene/maleic anhydride additive has low solubility to a cellulose acylate resin and so it is hard to produce a clear and low haze film when the additives are used.
At present, as in the above, technical development is assiduously desired in the art for a transparent protective film and an optically-compensatory film for use for polarizers, which is excellent in the durability of the optical properties thereof that control display performance, for which the production process is not complicated, which does not whiten, and for which the materials are not expensive, for a polarizer comprising the film, and for a liquid crystal display device comprising it.